Copper, copper alloys and several other valuable metals have been in use for thousands of years. Because of the importance of such metals, numerous entities have and continue to research ways to increase the efficiency and productivity of procurement methods. It is critical for mines to maximize efficiency when extracting metals from ore. Copper-containing ores are typically classified into two categories—oxidic and sulfidic ores. Oxidic ores (e.g., cuprite, malachite, and azurite) are found near the surface as they are oxidation products of the deeper secondary and primary sulfidic ores (e.g., chalcopyrite, bornite, and chalcocite). Due to the chemical nature of copper oxides and secondary sulfides, mines typically treat the ore with hydrometallurgical processes—i.e., heap leaching, solvent extraction, and electrowinning. Approximately 20% of the world's annual copper production is obtained through hydrometallurgical processes.
During hydrometallurgical processes, metal is extracted when the metal-containing material is leached in one of several ways. Leaching is typically accomplished by applying a lixiviant to a collection of ore. The most common lixiviant used in the mining industry is sulfuric acid (“H2SO4”) because it provides efficient and cost effective liberation of the metal from the ore. The leaching process can be a heap, dump, percolation or agitation leaching process. However, despite the leaching method, the intrinsic principles of leaching are the same: “1. [The process] . . . must dissolve the ore minerals rapidly enough to make commercial extraction possible[.] . . . [The process] should show chemical inertness toward gangue minerals . . . [because] [i]n situations where gangue minerals are attacked, an excessive amount of the lixiviant is consumed and the leach liquor fouled with impurities to an undesirable extent. 2. [The process] . . . must be cheap and readily obtainable in large quantities. 3. If possible, . . . [the process] should be such that it can be regenerated in the subsequent processes following leaching.” C. K. Gupta, T. K. Mukherjee, Hydrometallurgy in Extraction Processes, vol. 1. The underpinning characteristic of leaching is that regardless of the lixiviant used, it must be able to interact with the ore particles in a way that allows for transfer of the desired metal from the ore into a collected and then managed solution.
Heap leaching is a common method of leaching in hydrometallurgical processes; however, this method has disadvantages. When metal-containing material is piled into a heap and sprayed with a solution of dilute acid, significant time is required for the solution to percolate down through the heap before it can be collected and supplied to subsequent operations. The extraction process can require several days to months. Further issues arise when the fine particles in the heap accumulate between larger pieces of ore and decrease the speed of downward flow of the leaching solution or block the flow altogether. This results in channeling of the leachate (i.e., where the solution follows the path of least resistance through the heap), less contact with the packed fines, and a lower than expected concentration of metal in the resulting pregnant leaching solution (“PLS”). These accumulations can also lead to pooling of the metal-containing solution and ultimately a decrease in leaching yield as the valuable metal remains trapped in the heap.
To combat these issues, the ore can be agglomerated before applying the leaching solution. For example, agglomerating agents can be incorporated into the leaching solution and/or raffinate. Agglomerating agents function as binding agents for the smaller fines to the larger ore particles. This binding allows for more uniform percolation of the leaching solution through the heap. Such agglomerating agents can include strong acid and water combinations, anionic acrylamides, copolymers of acrylamide and acrylic acid, hydroxamated polymers, polyvinyl alcohols, ammonium cation and acrylamide-derived copolymers, and copolymers including combinations of poly(acrylamide), poly(acrylamide/sodium acrylate), poly(diallyldimethylammonium chloride), poly(acrylamide/diallyldimethylammonium chloride) and poly(diallyldimethylammonium chloride/vinyltrimethoxysilane) groups.
One drawback to the use of agglomerating agents is their limited ability to withstand acidic conditions, for example, from a sulfuric acid leaching solution. Breakdown of the agglomerating agents results in subsequent breakdown of the agglomerated particles. This quickly leads to the same issues as previously described, such as channeling and pooling within the heap. Channeling and pooling long have been a problem in heap leaching, and many have attempted to address such issues by introducing, for example, an antifoam, surfactant, or acid digestion agents. However, the mining industry has not widely adopted, for example, organic polymer type agglomeration agents for heap leaching because of their incompatibility with processes (e.g., solvent extraction, electrowinning) downstream from the leaching operation and added cost.
There remains a need for leaching aids, particularly in leaching solutions, and methods of using the leaching aids in a process for recovering metal from ore. According to various example aspects, the leaching aids are compatible in all aspects of a process including heap leaching, solvent extraction and electrowinning.